U.S. patent number 8,002,953 [Application Number 11/827,896] was granted by the patent office on 2011-08-23 for low-energy extractive distillation process for dehydration of aqueous ethanol.
This patent grant is currently assigned to AMT International Inc., CPC Corporation, Taiwan. Invention is credited to Tai-Ping Chang, Po-Sung Cheng, Jyh-Haur Hwang, Fu-Ming Lee, Tzong-Bin Lin, Hung-Chung Shen, Fong-Cheng Su, Lindsey Vuong, Kuang-Yeu Wu.
United States Patent |
8,002,953 |
Lee , et al. |
August 23, 2011 |
Low-energy extractive distillation process for dehydration of
aqueous ethanol
Abstract
An energy-efficient extractive distillation process for
producing anhydrous ethanol from aqueous/ethanol feeds containing
any range of ethanol employs an extractive distillation column
(EDC) that operates under no or greatly reduced liquid reflux
conditions. The EDC can be incorporated into an integrated process
for producing anhydrous ethanol used for gasoline blending from
fermentation broth. By using a high-boiling extractive distillation
solvent, no solvent is entrained by the vapor phase to the EDC
overhead stream, even under no liquid reflux conditions. The energy
requirement and severity of the EDC can be further improved by
limiting ethanol recovery in the EDC. In this partial ethanol
recovery design, ethanol which remains in the aqueous stream from
the EDC is recovered in a post-distillation column or the aqueous
stream is recycled to a front-end pre-distillation column where the
ethanol is readily recovered since the VLE curve for ethanol/water
is extremely favorable for distillation.
Inventors: |
Lee; Fu-Ming (Katy, TX), Wu;
Kuang-Yeu (Plano, TX), Vuong; Lindsey (Allen, TX),
Su; Fong-Cheng (Chiayi, TW), Lin; Tzong-Bin
(Chiayi, TW), Hwang; Jyh-Haur (Dali, TW),
Shen; Hung-Chung (Chiayi, TW), Cheng; Po-Sung
(Kaohsiung, TW), Chang; Tai-Ping (Cihtong Township,
TW) |
Assignee: |
AMT International Inc. (Plano,
TX)
CPC Corporation, Taiwan (Taipei, TW)
|
Family
ID: |
40252186 |
Appl.
No.: |
11/827,896 |
Filed: |
July 13, 2007 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20090014313 A1 |
Jan 15, 2009 |
|
Current U.S.
Class: |
203/19; 203/2;
203/87; 426/493; 203/75; 203/74; 203/99; 203/22; 203/62; 203/78;
203/23; 203/64; 203/DIG.19; 203/DIG.13; 426/492; 426/494; 203/60;
203/65; 203/1; 568/916 |
Current CPC
Class: |
B01D
3/002 (20130101); B01D 3/143 (20130101); C07C
29/84 (20130101); B01D 3/40 (20130101); C07C
29/84 (20130101); C07C 31/08 (20130101); Y02P
20/57 (20151101); Y10S 203/20 (20130101); Y02P
20/125 (20151101); Y10S 203/14 (20130101); Y02P
20/10 (20151101); Y02P 20/50 (20151101) |
Current International
Class: |
B01D
3/40 (20060101); B01D 3/42 (20060101); C07C
29/94 (20060101); C07C 29/84 (20060101); C12G
3/12 (20060101) |
Field of
Search: |
;203/1,2,19,22,23,58,60,62-65,73-75,78,80,87,98,99,DIG.8,DIG.13,DIG.19
;426/492-494 ;568/916 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
PCT/US2008/008223 Notification of transmittal of the International
Search Report and Written Opinion, Feb. 10, 2009. cited by other
.
Lee et al, "Two-Liquid-Phase extractive distillation for aromatics
recovery" Ind. Eng. Chem. Res., vol. 26, No. 3, (pp. 564-573)1987.
cited by other .
Drickamer & Hummel, "Application of experimental vapor-liquid
equilibria to an analysis of the operation of a commercial unit for
the purification of toluene from petroleum" Transactions of the Am.
Institute of Chem Eng. 41, pp. 607-629 (1945). cited by
other.
|
Primary Examiner: Manoharan; Virginia
Attorney, Agent or Firm: Cascio & Zervas
Claims
What is claimed is:
1. An improved extractive distillation (ED) process for dehydrating
an aqueous feedstock containing ethanol and water that comprises
the steps of: (a) introducing an aqueous feedstock comprising
ethanol and water into a middle portion of an extractive
distillation column (EDC); (b) introducing a high-boiling water
selective solvent into an upper portion of the EDC to contact the
aqueous feedstock under extractive distillation conditions to
produce a liquid bottoms stream that comprises water and
high-boiling water selective solvent and to produce a vaporous
overhead stream that comprises greater than 99.5 weight percent
ethanol, wherein the EDC is operated with a liquid
reflux-to-distillate ratio of less than about 0.5; (c) withdrawing
at least a portion of the vaporous overhead from the EDC as a
purified ethanol product; (d) feeding at least a portion of the
liquid bottoms stream of the EDC into a solvent recovery column
(SRC) to remove water therefrom and to yield a lean high-boiling
water selective solvent stream wherein step (a) comprises forming a
liquid bottoms stream comprising water, ethanol, and high-boiling
water selective solvent and in step (d) the SRC produces an
overhead stream comprising water and ethanol that is condensed to
form a condensate that is fed into a distillation column to yield a
liquid bottoms stream consisting essentially of water and an
overhead stream comprising ethanol and water which is recycled to
the EDC; and (e) recycling at least a portion of the lean
high-boiling water selective solvent stream into the upper portion
of the EDC.
2. The process of claim 1 wherein the aqueous feedstock, prior to
being introduced into the EDC, is initially preheated by the
vaporous overhead stream and subsequently preheated by the lean
high-boiling water selective solvent stream that is recycled to the
upper portion of the EDC.
3. The process of claim 1 wherein the aqueous feedstock comprises
85 to 95 wt % ethanol.
4. The process of claim 3 wherein the aqueous feedstock comprises
90 to 92 wt % ethanol.
5. The process of claim 1 wherein the high-boiling water selective
solvent is selected from the group consisting of glycerin,
tetraethylene glycol, ethylene glycol, diethylene glycol, and
mixtures thereof.
6. The process of claim 5 wherein the high-boiling water selective
solvent is glycerin and the purified ethanol product is food-grade
ethanol.
7. The process of claim 1 wherein step (b) comprises introducing a
high-boiling water selective solvent through an entry point at the
upper portion of the EDC wherein the temperature of the
high-boiling water, selective solvent is about 10.degree. C. to
20.degree. C. lower than that of the EDC at the entry point in
order to generate internal reflux.
8. The process of claim 1 wherein the lean high-boiling water
selective solvent which is removed from a bottom of the SRC has
less than about 0.5 wt % water.
9. The process of claim 1 wherein the EDC has a bottoms temperature
in a range of 160.degree. C. to 200.degree. C.
10. The process of claim 1 wherein the SRC has a bottoms
temperature in a range of 160.degree. C. to 200.degree. C.
11. The process of claim 1 wherein the SRC has an operating
pressure in a range of 50 to 500 mmHg (absolute).
12. The process of claim 11 wherein the SRC has an operating
pressure in a range of 150 to 300 mmHg (absolute).
13. The process of claim 1 wherein the amount of ethanol that is in
the liquid bottoms stream of the EDC is less than about 0.5 wt % of
the amount of ethanol that is in the aqueous feedstock.
14. The process of claim 1 wherein the amount of ethanol that is in
the liquid bottoms stream of the EDC is about 20 wt % of the amount
of ethanol that is in the aqueous feedstock.
15. The process of claim 1 wherein the amount of ethanol that is in
liquid bottoms stream of the EDC is no more than about 5 wt % of
the amount of ethanol that is in the aqueous feedstock.
16. An improved extractive distillation (ED) process for
dehydrating an aqueous feedstock containing ethanol and water that
comprises the steps of: (a) introducing an aqueous feedstock
comprising ethanol and water into a middle portion of an extractive
distillation column (EDC); (b) introducing a high-boiling ethanol
selective solvent into an upper portion of the EDC to contact the
aqueous feedstock under extractive distillation conditions to
produce a liquid bottoms stream consisting essentially of ethanol
and high-boiling ethanol selective solvent and a vaporous overhead
stream consisting essentially of water, wherein the EDC is operated
with a liquid reflux-to-distillate ratio of less than about 0.5;
(c) feeding at least a portion of the liquid bottoms stream of the
EDC into a solvent recovery column (SRC) to remove ethanol
therefrom and to yield a bottoms stream that comprises lean
high-boiling ethanol selective solvent and an overhead stream
whereby at least a portion of the overhead is withdrawn as a
purified ethanol product with higher than 99.5 wt % purity; and (d)
recycling at least a portion of the lean high-boiling ethanol
selective solvent from the bottoms stream of the SRC into the upper
portion of the EDC wherein the aqueous feedstock, prior to being
introduced into the EDC, is initially preheated by the vaporous
overhead vapor stream and subsequently preheated by lean
high-boiling ethanol selective solvent that is recycled to the
upper portion of the EDC.
17. The process of claim 16 wherein the aqueous feedstock comprises
85 to 95 wt % ethanol.
18. The process of claim 17 wherein the aqueous feedstock comprises
90 to 92 wt % ethanol.
19. The process of claim 16 wherein the high-boiling ethanol
selective solvent is selected from the group consisting of
2-phenyl-phenol, cumyl phenol, diisopropyl phenol, cyclohexyl
cyclohexanone, cyclohexyl cyclohexanol, methyl benzoate,
dipropylene glycol dibenzoate, trimellitic anhydride, and mixtures
thereof.
20. An improved extractive distillation (ED) process for
dehydrating an aqueous feedstock containing ethanol and water that
comprises the steps of: (a) introducing an aqueous feedstock
comprising ethanol and water into a middle portion of an extractive
distillation column (EDC); (b) introducing a high-boiling ethanol
selective solvent into an upper portion of the EDC to contact the
aqueous feedstock under extractive distillation conditions to
produce a liquid bottoms stream consisting essentially of ethanol
and high-boiling ethanol selective solvent and a vaporous overhead
stream consisting essentially of water, wherein the EDC is operated
with a liquid reflux-to-distillate ratio of less than about 0.5
wherein step (b) comprises introducing the high-boiling ethanol
selective solvent through an entry point at the upper portion of
the EDC wherein the temperature of the high-boiling ethanol
selective solvent is about 10.degree. C. to 20.degree. C. lower
than that of the EDC at the entry point in order to generate
internal reflux; (c) feeding at least a portion of the liquid
bottoms stream of the EDC into a solvent recovery column (SRC) to
remove ethanol therefrom and to yield a bottoms stream that
comprises lean high-boiling ethanol selective solvent and an
overhead stream whereby at least a portion of the overhead is
withdrawn as a purified ethanol product with higher than 99.5 wt %
purity; and (d) recycling at least a portion of the lean
high-boiling ethanol selective solvent from the bottoms stream of
the SRC into the upper portion of the EDC.
21. The process of claim 16 wherein the amount of ethanol that is
in the EDC overhead stream is less than about 0.5 wt % of the
amount of ethanol that is in the aqueous feedstock.
22. An improved extractive distillation (ED) process for
dehydrating an aqueous feedstock containing ethanol and water that
comprises the steps of: (a) introducing an aqueous feedstock
comprising ethanol and water into a middle portion of an extractive
distillation column (EDC); (b) introducing a high-boiling ethanol
selective solvent into an upper portion of the EDC to contact the
aqueous feedstock under extractive distillation conditions to
produce a liquid bottoms stream consisting essentially of ethanol
and high-boiling ethanol selective solvent and a vaporous overhead
stream consisting essentially of water and ethanol, wherein the EDC
is operated with a liquid reflux-to-distillate ratio of less than
about 0.5; (c) feeding at least a portion of the liquid bottoms
stream of the EDC into a solvent recovery column (SRC) to remove
ethanol therefrom and to yield a bottoms stream that comprises lean
high-boiling ethanol selective solvent and an overhead stream
whereby at least a portion of the overhead is withdrawn as a
purified ethanol product with higher than 99.5 wt % purity; (d)
recycling at least a portion of the lean high-boiling ethanol
selective solvent from the bottoms stream of the SRC into the upper
portion of the EDC; (e) feeding at least a portion of the vaporous
overhead stream from the EDC into a distillation column to produce
a second liquid bottoms stream consisting essentially of water and
a second overhead stream comprising ethanol and water; and (f)
recycling the second overhead stream into the EDC.
23. The process of claim 22 wherein the amount of ethanol that is
in the vaporous overhead stream of the EDC is about 20 wt % of the
amount of ethanol that is in the aqueous feedstock.
24. The process of claim 22 wherein the amount of ethanol in the
vaporous overhead stream of the EDC is no more than about 5 wt % of
the amount of ethanol that is in the aqueous feedstock.
25. An improved process for dehydrating an aqueous feedstock
containing ethanol and water that comprises the steps of: (a)
distilling an aqueous feedstock comprising ethanol and water in a
pre-distillation column to produce a first vaporous overhead
stream, a vaporous side-cut stream, and a first liquid bottoms
stream; (b) partially condensing the first vaporous overhead stream
in a first condenser to yield a condensate phase and a vapor phase;
(c) recycling the condensate phase from the first condenser into
the pre-distillation column as reflux; (d) introducing the vaporous
side-cut stream into a middle portion of an extractive distillation
column (EDC); (e) introducing a bottoms stream from a solvent
recovery column (SRC) which contains a high-boiling water selective
solvent into an upper portion of the EDC to contact the vaporous
side-cut stream under extractive distillation conditions to produce
a second vaporous overhead stream that comprises greater than 99.5
wt % ethanol and a second liquid bottoms stream comprising water,
ethanol and high-boiling water selective solvent wherein the EDC is
operated with a liquid reflux-to-distillate (R/D) ratio of less
than about 0.5; (f) introducing at least a portion of the second
liquid bottoms stream from the EDC that comprises water, ethanol
and high-boiling water selective solvent into the SRC to remove
water and ethanol from the high-boiling water selective solvent;
(g) recycling at least a portion of a bottoms stream of the SRC
that comprises high-boiling water selective solvent into an upper
portion of the EDC; (h) recycling a first portion of an overhead
condensate from the SRC as liquid reflux to the SRC and recycling a
second portion of the overhead condensate into the pre-distillation
column; and (i) withdrawing a bottom stream from the
pre-distillation column that consists essentially of water.
26. The process of claim 25 wherein the aqueous feedstock contains
1 to 12 wt % ethanol.
27. The process of claim 26 wherein the aqueous feedstock contains
10 to 12 wt % ethanol.
28. The process of claim 25 wherein step (a) comprises introducing
the aqueous feedstock into the pre-distillation column and wherein
the aqueous feedstock, prior to being introduced into the
pre-distillation column, is initially preheated by the second
vaporous overhead stream from the EDC, then preheated by the first
bottoms liquid stream of the pre-distillation column, and finally
preheated by the SRC bottoms stream comprising the high-boiling
water selective solvent.
29. The process of claim 25 wherein the first condenser operates at
a temperature in a range of about 60.degree. C. to 65.degree. C. to
condense primarily ethanol and in a second condenser that operates
at a lower temperature to condense light impurities including
methanol and esters.
30. The process of claim 25 wherein the EDC is operated with no
liquid reflux at the top of the column.
31. The process of claim 25 wherein the high-boiling water
selective solvent is selected from the group consisting of
glycerin, tetraethylene glycol, ethylene glycol, diethylene glycol,
and mixtures thereof.
32. The process of claim 31 wherein the high-boiling water
selective solvent is glycerin and step (e) produces a second
vaporous overhead stream that comprises food-grade ethanol.
33. The process of claim 25 wherein step (e) comprises introducing
a bottoms stream from the SRC to an upper portion of the EDC
wherein the temperature of the bottoms stream is about 10.degree.
C. to 20.degree. C. lower than that of the EDC at the entry point
in order to generate internal reflux.
34. The process of claim 25 wherein the amount of ethanol in the
EDC bottoms stream is about 20 wt % of the ethanol in the aqueous
feedstock.
35. The process of claim 34 wherein the amount of ethanol in the
EDC bottoms stream is no more than about 5 wt % of the ethanol in
the aqueous feedstock.
36. The process of claim 25 wherein the high-boiling water
selective solvent from the bottoms of the SRC comprises less than
about 0.5 wt % water.
37. The process of claim 25 wherein the EDC has a bottoms
temperature in a range of 160.degree. C. to 200.degree. C.
38. The process of claim 25 wherein the SRC has a bottoms
temperature in a range of 160.degree. C. to 200.degree. C.
39. The process of claim 25 wherein the SRC has an operating
pressure in a range of 50 to 500 mmHg (absolute).
40. The process of claim 39 wherein the SRC has an operating
pressure in a range of 150 to 300 mmHg (absolute).
41. An improved process for dehydrating an aqueous feedstock
containing ethanol and water that comprises the steps of: (a)
distilling an aqueous feedstock comprising ethanol and water in a
pre-distillation column to produce a first vaporous overhead
stream, a vaporous side-cut stream, and a first liquid bottoms
stream; (b) partially condensing the first vaporous overhead stream
in a first condenser to yield a condensate phase and a vapor phase;
(c) recycling the condensate phase from the first condenser into
the pre-distillation column as reflux; (d) introducing the vaporous
side-cut stream into a middle portion of an extractive distillation
column (EDC); (e) introducing a bottoms stream from a solvent
recovery column (SRC) which contains a high-boiling ethanol
selective solvent into an upper portion of the EDC to contact the
vaporous side-cut stream under extractive distillation conditions
to produce a second vaporous overhead stream that comprises water
and ethanol and a second liquid bottom streams that comprises
ethanol and the high-boiling ethanol selective solvent wherein the
EDC is operated with a liquid reflux-to-distillate ratio of less
than about 0.5; (f) recycling at least a portion of the second
vaporous overhead stream from the EDC in the form of condensate
into the pre-distillation column; (g) introducing at least a
portion of the second liquid bottoms stream from the EDC that
comprises ethanol and high-boiling ethanol selective solvent into
the SRC to yield an overhead ethanol product that comprises greater
than 99.5 weight percent ethanol and a bottoms stream that
comprises high-boiling ethanol selective solvent; (h) recycling at
least a portion of the SRC bottoms stream comprising the
high-boiling ethanol selective solvent into an upper portion of the
EDC; (i) recycling a portion of the ethanol product in the form of
condensate into the SRC as reflux; and (j) withdrawing a bottoms
stream from the pre-distillation column that consists essentially
of water.
42. The process of claim 41 wherein the aqueous feedstock contains
1 to 12 wt % ethanol.
43. The process of claim 42 wherein the aqueous feedstock contains
10 to 12 wt % ethanol.
44. The process of claim 41 wherein step (a) comprises introducing
the aqueous feedstock into the pre-distillation column and wherein
the aqueous feedstock, prior to being introduced into the
pre-distillation column, is initially preheated by the first liquid
bottoms stream from the pre-distillation column, then preheated by
the second vaporous overhead stream from the EDC, and finally
preheated by the bottoms stream containing the high-boiling ethanol
selective solvent from the SRC.
45. The process of claim 41 wherein the first condenser operates at
a temperature in a range of about 60.degree. C. to 65.degree. C. to
condense primarily ethanol and in a second condenser that operates
at a lower temperature to condense light impurities including
methanol and esters.
46. The process of claim 41 wherein the EDC is operated with no
liquid reflux at the top of the column.
47. The process of claim 41 wherein the high-boiling ethanol
selective solvent is selected from the group consisting of
2-phenyl-phenol, cumyl phenol, diisopropyl phenol, cyclohexyl
cyclohexanone, cyclohexyl cyclohexanol, methyl benzoate,
dipropylene glycol dibenzoate, trimellitic anhydride, and mixtures
thereof.
48. The process of claim 41 wherein step (d) comprises introducing
a high-boiling ethanol selective solvent to the EDC wherein the
temperature of the high-boiling ethanol selective solvent is about
10.degree. C. to 20.degree. C. lower than that of the EDC at the
entry point to generate internal reflux.
49. The process of claim 41 wherein the amount of ethanol in the
EDC overhead stream is about 20 wt % of the amount of ethanol in
the aqueous feedstock.
50. The process of claim 49 wherein the amount of ethanol in the
EDC overhead stream is less than about 5 wt % of amount of the
ethanol in the aqueous feedstock to the EDC.
Description
FIELD OF THE INVENTION
The present invention relates to an improved, energy efficient
extractive distillation process for recovering anhydrous (99.5+ wt
%) ethanol, that is suitable for gasoline blending, from an aqueous
ethanol feedstock, including fermentation broth.
BACKGROUND OF THE INVENTION
Higher oil prices and more stringent environmental regulations have
been the impetus for allocating greater resources into research and
development for alternate and renewable fuels worldwide. An
important development in this regard is the use of ethanol as the
blending stock for gasoline. Burning ethanol instead of gasoline
could reduce carbon emissions by more than 80% and completely
eliminate the release of acid-rain-causing sulfur dioxide. The U.S.
Department of Energy (DOE) predicts that ethanol could reduce
gasoline consumption by 30% in the United Sates by 2030. A recent
DOE report concluded that, in terms of key energy and environmental
benefits, fermentation ethanol is clearly superior to
petroleum-based fuels, and future cellulosic-based ethanol will be
even better.
A major drawback of using ethanol is that fermentation yields
dilute aqueous ethanol mixtures that contain only 10-12 wt %
ethanol. Current dehydration strategies, for producing anhydrous
(99.5 wt %) ethanol that is suitable for gasoline blending, are
energy intensive and employ conventional distillation and a
subsequent step to break the ethanol-water azeotrope. As compared
to the energy value of ethanol which is 21,400 KJ/L, conventional
distillation requires approximately 6,400 KJ/L to concentrate
fermentation ethanol to 95.6 wt % ethanol, which is the azeotropic
composition. Even improved distillation schemes that employ heat
integration still require 5,500 KJ/L to produce the azeotropic
composition. Thereafter, concentrating the ethanol from the
azeotropic point to produce anhydrous ethanol requires more energy.
As is apparent, despite the benefits of using ethanol, the energy
costs associated with dehydrating ethanol present serious economic
impediments for using ethanol produced by fermentation as a
gasoline blending stock or as an engine fuel.
The major commercial methods for concentrating ethanol beyond the
azeotropic composition for producing anhydrous ethanol are: (1)
azeotropic distillation, (2) molecular sieves adsorption, and (3)
extractive distillation. A fourth method known as membrane
separation which uses zeolitic or polymer membranes to break the
azeotrope is largely in the developmental stages.
In azeotropic distillation, a volatile entrainer is added to an
aqueous ethanol feed mixture. The entrainer modifies the activity
coefficients of the water and ethanol being separated and forms an
azeotrope with the water to be taken overhead as the process yields
anhydrous ethanol. Pentane, benzene, diethyl ether, and gasoline
have been disclosed as suitable entrainers. See, U.S. Pat. No.
3,575,818 to West, U.S. Pat. No. 2,012,199 to McElroy, U.S. Pat.
No. 2,371,010 to Wolfner, and Black et al., "Extractive and
Azeotropic Distillation," Am. Chem. Soc. Advances In Chemistry
Series No. 115, p. 64, 1972. The major disadvantage of the
azeotropic method is that the ethanol feed has to be
pre-concentrated to near 95 wt %, the azeotropic composition, which
is an energy intensive process in itself.
Adsorptive separation processes for separating of ethanol from
water are typically batch processes, with respect to the adsorbents
used, that entail an adsorption cycle and a separate desorption
cycle. Various adsorptive separation techniques are described in,
for example, U.S. Pat. No. 4,273,621 to Fornoff, U.S. Pat. No.
4,407,662 to Ginder, U.S. Pat. No. 4,465,875 to Greenbank et al.,
U.S. Pat. No. 4,287,089 to Convers et al., U.S. Pat. No. 4,277,635
to Oulman, U.S. Pat. No. 4,382,001 to Kulprathipanja et al., U.S.
Pat. No. 5,030,775 to Sircar, U.S. Pat. No. 4,343,623 to
Kulprathipanja et al., U.S. Pat. No. 4,319,058 to Kulprathipanja et
al, U.S. Pat. No. 5,766,895 to Valkanas et al, U.S. Pat. No.
4,359,593 to Feldman and U.S. Pat. No. 2,137,605 to Derr. Although
adsorption methods are very selective in removing water or ethanol,
the associated high heat energy requirements, high operating costs,
limited capacities, and uncertainty in the lengths of adsorbent
lives are major drawbacks toward commercial operations.
Finally, in extractive distillation (ED) a high-boiling, polar,
nonvolatile solvent is added to the upper portion of an extractive
distillation column (EDC), while the feed containing ethanol and
water is fed the middle or lower portion of the EDC, which is below
the solvent entry point. Depending upon the properties of the
solvent, the descending nonvolatile solvent preferentially extracts
water or ethanol from the ascending vapor stream thereby
eliminating the ethanol-water azeotrope and producing purified
ethanol or water from the overhead of EDC. A portion of the EDC
overhead stream is recycled to the top of the EDC as reflux.
Saturated solvent that is rich in water or ethanol is then
withdrawn from the bottom of EDC and transferred to the middle
portion of a solvent recovery column (SRC). Water or ethanol in the
saturated solvent is stripped from the solvent by heat from a SRC
reboiler and then recovered from the overhead stream of the SRC as
purified water or ethanol. Again, a portion of the overhead stream
is recycled to the top of SRC as liquid reflux. Lean solvent from
the bottom of SRC is circulated back to the EDC as the solvent
feed.
An ED process for dehydrating aqueous ethanol using glycerin as the
ED solvent was disclosed in U.S. Pat. No. 1,469,447 to Schneible.
Subsequently, other ED solvents considered include: ethoxyethanol
and butoxyethanol (U.S. Pat. No. 2,559,519 to Smith et al.), butyl,
amyl or hexyl alcohols (U.S. Pat. No. 2,591,671 to Catterall),
gasoline components (U.S. Pat. No. 2,591,672 to Catterall),
sulfuric acid, acetone or furfural (U.S. Pat. No. 2,901,404 to
Kirshenbaum et al.), 2-phenyl phenol, or mixtures of 2-phenyl
phenol and cumyl phenol (U.S. Pat. No. 4,428,798 Zudkevitch et
al.), cyclohexylcyclohexanone or cyclohexylcyclohexanol (U.S. Pat.
No. 4,455,198 to Zudkevitch et al.), methyl benzoate, mixture of
methyl benzoate and trimellitic anhydride, and mixture of
dipropylene glycol dibenzoate, ethyl salicylate and resorcinaol
(U.S. Pat. No. 4,631,115 to Berg et al.), hexahydrophthalic
anhydride, mixture of methyl tetrahydrophthalic anhydride and
pentanol-1, and mixture of trimellitic anhydride, ethyl salicylate
and resorcinol (U.S. Pat. No. 4,654,123 to Berg et al.); and
diaminobutane, 1,3-diaminopentane, diethylenetriamine, and
hexachlorobutadiene (U.S. Pat. No. 6,375,807 to Nieuwoudt). The ED
solvents disclosed in these patents were said to be particularly
selective in separating ethanol and water, but without regard to
their practical applicability to ED processes with respect to other
important solvent properties such as solvent thermal stability,
toxicity, boiling point, etc. These patents also did not address
energy and related economics issues related to ED processes.
In order to reduce the energy requirements in ED processes, U.S.
Pat. No. 4,400,241 to Braithwaite et al. proposed adding
alkali-metal or alkaline-earth metal salts to a polyhydric alcohol
solvent to enhance the ED solvent performance. The preferred
systems include (1) sodium tetraborate that is added to ethylene
glycol and (2) dipotassium phosphate that is added glycerin.
Other approaches to reducing energy requirements featured improved
process designs to recovery energy such as that in U.S. Pat. No.
4,349,416 to Brandt et al. where a first side stream is withdrawn
from the EDC, passed in heat exchange with the bottoms from the EDC
en route to the SRC and returned to the EDC at a point below the
point of the side stream. A second side stream from the EDC is also
withdrawn, passed in heat exchange with the bottoms of the SRC and
returned to the EDC.
U.S. Pat. No. 4,559,109 to Lee et al. disclosed another approach
whereby aqueous ethanol containing 10-12 wt % ethanol is first
converted to an 85-90 wt % concentrated vapor in a front-end
distillation column which is then fed to an EDC. The front-end
distillation column represents the water-rich (lower) portion of
the fractionator, while the EDC represents the ethanol-rich (upper)
portion of the fractionator. Extractive solvent is added only to
the EDC (ethanol-rich portion of the fractionator) where the
vapor-liquid equilibrium (VLE) curve is very unfavorable for
distillation. The solvent is said to eliminate the binary
ethanol-water azeotrope and modify the shape of the ethanol-rich
portion of the VLE curves favorably for distillation. The
ethanol-water saturated solvent is then completely removed via a
rich solvent bottoms stream from the EDC and fed to a SRC (solvent
stripper). The vaporous overhead stream of the SRC is said to be
recycled directly to an upper portion of the front-end distillation
column while lean solvent from the bottom of the SRC is fed to the
top of the EDC. In a preferred application of the process, the rich
solvent bottoms stream from the EDC which is fed to the SRC
contains more than 40 wt % of the ethanol that is present in the
vaporous feed to the EDC from the front-end distillation column (or
simply more than 40 wt % of the ethanol in the feed to the
front-end distillation column).
In practice the techniques described in U.S. Pat. No. 4,559,109 are
neither energy nor cost effective because the rich solvent stream,
which contains 40 wt % of the feed ethanol, must be vaporized in
the SRC and then again in the pre-distillation column. Initially
the rich solvent is vaporized in the SRC to separate the solvent
from the water and the ethanol, but it has been demonstrated that
the water and the ethanol cannot be directly recycled to the
pre-distillation column in the form of vapor as stated in the
patent. The reason is that the SRC is normally operated under
reduced pressures in order to lower the bottoms temperature so as
to minimize solvent decomposition. When ethylene glycol is employed
as the ED solvent, as shown in FIGS. 5 and 6 of the patent, the SRC
should be operated at substantially lowered pressures of about 150
mmHg at the top in order to keep the bottoms temperature below
180.degree. C. The pre-distillation column, on the other hand, is
operated at higher pressures than that of the EDC to allow the
vapor stream to be fed from the overhead of the pre-distillation
column to the EDC, which is preferred procedure as described in the
patent. Because of these constraints, the overhead vapor stream of
the SRC has to be condensed before it can be pumped into the higher
pressured pre-distillation column, where the ethanol content in the
condensed SRC recycle stream is re-vaporized. Another problem
associated with the process depicted in U.S. Pat. No. 4,559,109 is
that there is no liquid reflux for the SRC which causes the upper
trays in the SCR to run dry.
Other energy saving ED techniques employ a combination of the basic
distillation method along with (1) multi-effect distillation, (2)
conventional overhead-to-reboiler heat pumped distillation, (3)
azeotropic and extractive distillation, or (4) fermentative
production of volatile compounds, which is disclosed in U.S. Pat.
No. 4,961,826 to Grethlein et al. All of the energy saving methods
in the prior art for the ED process or combination process schemes
associated ED methods are either too limited in scope for energy
savings or too complicated for practical applications.
SUMMARY OF THE INVENTION
The present invention provides a simple, energy efficient
extractive distillation (ED) process for producing anhydrous
ethanol (99.5+ wt % ethanol) from aqueous ethanol feeds containing
any range of ethanol. The improved ED technique is particularly
suited for incorporation into an integrated process for producing
anhydrous ethanol, which is suitable for gasoline blending, from
fermentation broth which typically contains 1 to 12 wt % and more
preferably 10 to 12 wt % ethanol.
The invention is based in part on the discovery that the liquid
reflux within an extractive distillation column (EDC) has
essentially no effect with respect to separating ethanol and water;
rather, when liquid reflux is employed, it functions primarily to
knock down the entrained solvent which is otherwise carried by the
vapor phase into the EDC overhead stream. A corollary to operating
the EDC under no liquid reflux or greatly reduced liquid reflux
conditions is that the energy input to the EDC is substantially
reduced. An aspect of the invention is that by using a high-boiling
ED solvent, no solvent is entrained by the vapor phase to the EDC
overhead stream, even under no liquid reflux conditions. A feature
of the invention is that the energy requirement and severity of the
EDC can be further improved by modifying the process to limit
ethanol recovery in the EDC. In this partial ethanol recovery
design, ethanol which remains in the aqueous stream from the EDC is
recovered in a post-distillation column or the aqueous stream is
recycled to a front-end pre-distillation column where the ethanol
is readily recovered since the water-rich portion of the
vapor-liquid equilibrium curve for ethanol/water is extremely
favorable for distillation.
In one embodiment, the invention is directed to an improved
extractive distillation (ED) process for dehydrating an aqueous
feedstock containing ethanol and water that includes the steps of:
(a) introducing an aqueous feedstock comprising ethanol and water
into a middle portion of an extractive distillation column (EDC);
(b) introducing a high-boiling water selective solvent into an
upper portion of the EDC to contact the aqueous feedstock under
extractive distillation conditions to produce a liquid bottoms
stream that comprises water and high-boiling water selective
solvent and to produce a vaporous overhead stream that comprises
greater than 99.5 weight percent ethanol, wherein the EDC is
operated with a liquid reflux-to-distillate ratio of less than
about 0.5; (c) withdrawing at least a portion of the vaporous
overhead from the EDC as a purified ethanol product; (d) feeding at
least a portion of the liquid bottoms stream of the EDC into a
solvent recovery column (SRC) to remove water therefrom and to
yield a lean high-boiling water selective solvent stream; and (e)
recycling at least a portion of the lean high-boiling water
selective solvent stream into an upper portion of the EDC.
In another embodiment, the invention is directed to an improved
extractive distillation (ED) process for dehydrating an aqueous
feedstock containing ethanol and water that includes the steps of:
(a) introducing an aqueous feedstock comprising ethanol and water
into a middle portion of an extractive distillation column (EDC);
(b) introducing a high-boiling ethanol selective solvent into an
upper portion of the EDC to contact the aqueous feedstock under
extractive distillation conditions to produce a liquid bottoms
stream consisting essentially of ethanol and high-boiling selective
solvent and a vaporous overhead stream comprising essentially of
water, wherein the EDC is operated with a liquid
reflux-to-distillate ratio of less than about 0.5; (c) feeding at
least a portion of the liquid bottoms stream of the EDC into a
solvent recovery column (SRC) to remove ethanol therefrom and to
yield a bottoms stream that comprises lean high-boiling ethanol
selective solvent and an overhead stream whereby at least a portion
of the overhead is withdrawn as a purified ethanol product with
higher than 99.5 wt % purity; and (d) recycling at least a portion
of the lean high-boiling ethanol selective solvent from the bottoms
stream of the SRC into the upper portion of the EDC.
In a further embodiment, the invention is directed to an improved
process for dehydrating an aqueous feedstock containing ethanol and
water that includes the steps of: (a) distilling an aqueous
feedstock comprising ethanol and water in a pre-distillation column
to produce a first vaporous overhead stream, a vaporous side-cut
stream, and a first liquid bottoms stream; (b) partially condensing
the first vaporous overhead stream in a first condenser to yield a
condensate phase and a vapor phase; (c) recycling the condensate
phase from the first condenser into the pre-distillation column as
reflux; (d) introducing the vaporous side-cut stream into a middle
portion of an extractive distillation column (EDC); (e) introducing
a bottoms stream from a solvent recovery column (SRC) which
contains a high-boiling water selective solvent into an upper
portion of the EDC to contact the vaporous side-cut stream under
extractive distillation conditions to produce a second vaporous
overhead stream that comprises greater than 99.5 wt % ethanol and a
second liquid bottoms stream comprising water, ethanol and
high-boiling water selective solvent wherein the EDC is operated
with a liquid reflux-to-distillate (R/D) ratio of less than about
0.5; (f) introducing at least a portion of the second liquid
bottoms stream from the EDC that comprises water, ethanol and
high-boiling water selective solvent into the SRC to remove water
and ethanol from the high-boiling water selective solvent; (g)
recycling at least a portion of a bottoms stream of the SRC that
comprises high-boiling water selective solvent into an upper
portion of the EDC; (h) recycling a first portion of an overhead
condensate from the SRC as liquid reflux to the SRC and recycling a
second portion of the overhead condensate into the pre-distillation
column; and (i) withdrawing a bottom stream from the
pre-distillation column that consists essentially of water.
In yet another embodiment, the invention is directed to an improved
process for dehydrating an aqueous feedstock containing ethanol and
water that includes the steps of: (a) distilling an aqueous
feedstock comprising ethanol and water in a pre-distillation column
to produce a first vaporous overhead stream, a vaporous side-cut
stream, and a first liquid bottoms stream; (b) partially condensing
the first vaporous overhead stream in a first condenser to yield a
condensate phase and a vapor phase; (c) recycling the condensate
phase from the first condenser into the pre-distillation column as
reflux; (d) introducing the vaporous side-cut stream into a middle
portion of an extractive distillation column (EDC); (e) introducing
a bottoms stream from a solvent recovery column (SRC) which
contains a high-boiling ethanol selective solvent into an upper
portion of the EDC to contact the vaporous side-cut stream under
extractive distillation conditions to produce a second vaporous
overhead stream that comprises water and ethanol and a second
liquid bottom streams that comprises ethanol and the high-boiling
ethanol selective solvent wherein the EDC is operated with a liquid
reflux-to-distillate ratio of less than about 0.5; (f) recycling at
least a portion of the second vaporous overhead stream from the EDC
in the form of condensate into the pre-distillation column; (g)
introducing at least a portion of the second liquid bottoms stream
from the EDC that comprises ethanol and high-boiling ethanol
selective solvent into the SRC to yield an overhead ethanol product
that comprises greater than 99.5 weight percent ethanol and a
bottoms stream that comprises high-boiling ethanol selective
solvent; (h) recycling at least a portion of the SRC bottoms stream
comprising the high-boiling ethanol selective solvent into an upper
portion of the EDC; (i) recycling a portion of the ethanol product
in the form of condensate into the SRC as reflux; and (j)
withdrawing a bottoms stream from the pre-distillation column that
consists essentially of water.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a low-energy extractive distillation process for
the dehydration of aqueous ethanol;
FIGS. 2 and 3 illustrate extractive distillation processes having a
post-distillation column for dehydrating aqueous ethanol using
high-boiling water selective solvents and using high-boiling
ethanol selective solvents, respectively; and
FIGS. 4 and 5 illustrate extractive distillation processes having
pre-distillation column for dehydrating fermentation broth using
high-boiling water selective solvents and high-boiling ethanol
selective solvents, respectively.
DETAILED DESCRIPTION OF THE INVENTION
In conventional distillation, the liquid reflux at the top of
distillation column governs the amount of liquid phase that is in
equilibrium with the rising vapor phase at each of the contacting
stages in the column. Depending on the feed composition, the
product requirements, and the vapor liquid equilibrium curve (VLE)
of the key components being separated, the reflux to distillate
(R/D) ratio may range from 1 to 10 or even higher and typically
ranges from 2 to 5. Therefore, the R/D ratio is the major operating
variable that determines not only the product purity and recovery
but the process energy requirements as well since the liquid reflux
has to be vaporized in the column.
The present invention is based, in part, on the discovery that, in
extractive distillation (ED), the ED solvent that is fed to the
upper portion (near the top) of an extraction distillation column
(EDC) can serve as the liquid phase as it descends down the column
and is in equilibrium with the ascending vapor phase at each stage
in the column. Hence, the liquid reflux which dictates the major
energy requirement of the distillation column has essentially no
functional purpose in the EDC.
Low-Energy Extractive Distillation Scheme for Dehydration of
Aqueous Ethanol
Referring to the process shown in FIG. 1, an aqueous ethanol feed
is fed via line or stream 1 to the middle portion of extractive
distillation column (EDC) 201. The feed contains typically 10 to 95
wt % ethanol, preferably 85 to 95 wt % ethanol, and more preferably
90 to 92 wt % ethanol. In a preferred but optional embodiment, feed
stream 1 is initially heated by the EDC overhead vapor in condenser
204 before being partial or totally vaporized by the lean solvent
feed to the EDC in cooler 203. Lean solvent from the bottom of
solvent recovery column (SRC) 202 is fed via lines 13 and 2 into an
upper portion of EDC 201 after being cooled in cooler 203. The lean
solvent entry point in EDC 201 is selected at a location such that
there are a few trays above the solvent tray to essentially
eliminate entrained solvent that is carried over to the EDC
overhead. Overhead vapor exiting EDC 201 via line 3 is condensed in
condenser 204 and the condensate is transferred into accumulator
206. Preferably, no liquid reflux is recycled to the top of EDC 201
although a reflux-to-distillate (R/D) ratio ranging from 0 to less
than 0.5 can be used for non-functional purposes, such as for
knocking down entrained solvent from the overhead vapor stream 3.
If necessary, the lean solvent temperature can be maintained
20.degree. C. and preferably 10.degree. C. below the EDC
temperature near the solvent tray in order to generate internal
reflux without expending energy.
If the high-boiling ED solvent that is used preferentially extracts
water in EDC 201, ethanol product with 99.5+ wt % purity is
withdrawn from accumulator 206 by pump 208 through line 4. On the
other hand, if the high-boiling ED solvent that is used
preferentially extracts ethanol in EDC 201, a stream consisting
essentially of water is withdrawn from accumulator 206 by pump 208
through line 4. To ensure that the overhead stream 3 is not
contaminated with entrained solvent when the system is operating
under no reflux conditions, the preferred high-boiling ED water
selective solvent that is employed for preferentially extracting
water in EDC 201 preferably has a boiling point of at least
200.degree. C. and is selected from glycerin, ethylene glycol,
diethylene glycol, triethylene glycol, tetraethylene glycol,
trimethylene glycol, 1,4 butanediol, and combinations thereof. On
the basis of their boiling points and selectivity, the preferred
solvents are glycerin, tetraethylene glycol, and ethylene glycol,
and diethylene glycol.
Preferred high boiling ED ethanol selective solvent for
preferentially extracting ethanol in the EDC preferably has a
boiling point of at least 200.degree. C. and is selected from of
C.sub.6+ phenols (including 2-phenyl-phenol, cumyl phenol,
diisopropyl phenol and mixtures thereof), cyclic C.sub.7 ketones
(including cyclohexyl cyclohexanone), cyclic C.sub.8 alcohols
(including cyclohexyl cyclohexanol), methyl benzoate, dipropylene
glycol dibenzoate, trimellitic anhydride, and mixtures thereof.
Solvents that contain sulfur should not be used.
To keep the bottom temperature of EDC 201 at less than 200.degree.
C. and preferably in the range of 160.degree. C. to 180.degree. C.
in order to minimize solvent decomposition, EDC 201 is operated
under slightly positive pressures. Depending upon the solvent
function for extracting water or ethanol, rich solvent saturated
with ethanol or water, respectively, is withdrawn from the bottom
of EDC 201 with pump 211 via line 5. A portion of the bottoms is
passed through reboiler 210 and recycled to the bottom of EDC 201
via line 6 to maintain the vapor flow in the column. Since there is
minimal or no reflux in EDC 201, the energy requirement for
reboiler 210 is substantially reduced.
Rich solvent stream from EDC 201 in the form of a partially
vaporized mixture is fed via lines 5 and 7 to the middle portion of
solvent recovery column (SRC) 202, which is operated at reduced
pressures (vacuum) to maintain the bottom temperature at below
200.degree. C. and preferably in the range of 160.degree. C. to
180.degree. C. in order to minimize solvent decomposition. A vapor
stream containing essentially pure water or ethanol, depending upon
the solvent function, exits the top of SRC 202 via line 8 and is
condensed in condenser 205. Pressure in condenser 205 as well as
the top of SRC 202 typically ranges from 50 to 500 mmHg (absolute)
and preferably from 150 to 300 mmHg (absolute), depending upon the
boiling point of the solvent. The SRC 202 overhead is connected to
a vacuum source (not shown), which can be a vacuum pump or steam
ejector, through line 9. The condensate from condenser 205 is
transferred via line 10 to accumulator 207, where a portion of the
condensate is recycled through pump 209 to the top of SRC 202 via
line 11 as a liquid reflux to provide liquid flow for the upper
portion of SRC 202. The remaining portion of the condensate is
withdrawn from accumulator 207 with pump 209 via line 12 in the
form of water or the anhydrous ethanol product, again, depending on
the solvent function.
In the meantime, lean solvent is withdrawn from the bottom of SRC
202 with pump 213 via line 13. A portion of this lean solvent is
heated in reboiler 212 and recycled to the bottom of SRC 202 via
line 14 to maintain the bottoms temperature in the desired
temperature range. In the case where the anhydrous ethanol product
is produced from the overhead of EDC 201, maintaining the bottoms
temperature within these ranges keeps the water content in the lean
solvent below 1 wt % and preferably in the range of 0.2 to 0.5 wt
%. Higher water content in the lean solvent would lower the ethanol
product purity since the lean solvent is fed to near the top of EDC
201.
Extractive Distillation Scheme with a Post-Distillation Column for
Dehydration of Aqueous Ethanol Using Water Selective Solvents
FIG. 2 shows an integrated process for dehydrating aqueous ethanol
that employs a system which includes an EDC 221, SRC 222, and a
post-distillation column (Post-DC) 223 wherein the ED solvent used
preferentially extracts water in the EDC. Again, aqueous ethanol
feed containing 10 to 95 wt % ethanol, preferably 85 to 95 wt %
ethanol, and more preferably 90 to 92 wt % ethanol is fed into EDC
221 via lines or streams 21 and 22. In a preferred but optional
embodiment, feed stream 21 is initially heated by the EDC overhead
vapor which is circulated through condenser 225 before being
partial or totally vaporized by the lean solvent feed that is
circulated through cooler 224.
Lean solvent from the bottom of SRC 222 is fed via lines 33 and 23
into an upper portion of EDC 221 after being cooled in cooler 224.
The lean solvent entry point in EDC 221 is selected at a location
such that there are a few trays above the solvent tray to
essentially eliminate entrained solvent that is carried over to the
EDC overhead. Overhead vapor exiting EDC 221 via line 24 is
condensed in condenser 225 and the condensate is transferred into
accumulator 228. Preferably, no liquid reflux is recycled to the
top of EDC 221 although a reflux-to-distillate ratio ranging from 0
to less than 0.5 can be used for non-functional purposes, such as
for knocking down entrained solvent from the overhead vapor stream
24. If necessary, the lean solvent temperature can be maintained
20.degree. C. and preferably 10.degree. C. below the EDC
temperature near the solvent tray in order to generate internal
reflux without expending energy. Ethanol product with 99.5+ wt %
purity is withdrawn from accumulator 228 by pump 231 through line
25.
To keep the bottom temperature of EDC 221 at less than 200.degree.
C. and preferably at less than 180.degree. C. in order to minimize
solvent decomposition, EDC 221 is operated under slightly positive
pressures. Rich solvent saturated with water is withdrawn from the
bottom of EDC 221 with pump 237 via line 26. A portion of the
bottoms is passed through reboiler 234 and recycled to the bottom
of EDC 221 via line 27 to maintain the vapor flow in the
column.
In this process, EDC 221 is configured to produce an ethanol
product with 99.5+ wt % purity in the overhead and to allow a
certain amount of ethanol in the bottoms. EDC 221 operates without
liquid reflux (or minimal reflux) and requires fewer separation
stages and a lower solvent-to-feed (S/F) ratio. The EDC bottoms
contains preferably no more than 20 wt % and more preferably no
more than 5 wt % of the ethanol that is in the feed to EDC 221. The
preferred operating conditions of EDC 221 and SRC 222 are similar
to those disclosed for EDC 201 and SRC 202, respectively, for the
process shown in FIG. 1.
Rich solvent stream from EDC 221 in the form of a partially
vaporized mixture is fed via lines 26 and 28 to the middle portion
of SRC 222, which is operated at reduced pressures (vacuum) to
maintain the bottom temperature at below 200.degree. C. and
preferably in the range of 160.degree. C. to 180.degree. C. in
order to minimize solvent decomposition. Overhead aqueous vapor
stream from SRC 222, which contains water and a certain amount of
ethanol, is transferred via line 29 to condenser 226 under reduced
pressure (vacuum), where the condensate is transferred to
accumulator 229 via line 31. SRC 222 overhead is connected to a
vacuum source (not shown) such as a vacuum pump or steam injector
through line 30. At least a portion of the condensate is recycled
to SRC 222 as liquid reflux by pump 232 via line 32. The remaining
portion of the condensate is fed to Post-DC 223 via line 35 to
recover ethanol. Based on VLE curves for water and ethanol
mixtures, Post-DC 223 can be operated favorably to produce pure
water in the bottoms and a vapor mixture of water and ethanol in
the overhead stream, which exits via line 37; at least a portion of
the overhead vapor in line 38 is condensed in condenser 227 and
sent to accumulator 230 via line 40 and then recycled to Post-DC
223 as liquid reflux through pump 233. In order to recycle the
remaining portion of overhead vapor stream from Post-DC 223 to EDC
221 via lines 39 and 22 to recover ethanol, Post-DC 223 is operated
at higher pressures than that of EDC 221. The bottoms stream of
Post-DC 223 containing pure water is withdrawn by pump 239 via line
36. A portion of this bottoms stream is heated by reboiler 236 and
recycled to the bottom of Post-DC 223 for generating the vapor flow
whereas the rest of stream 36 is withdrawn via line 41 for
disposal.
In the meantime, lean solvent is withdrawn from the bottom of SRC
222 with pump 238 via line 33. A portion of this lean solvent is
heated in reboiler 235 and recycled to the bottom of SRC 222 via
line 34 to maintain the bottoms temperature in the desired
temperature range.
Extractive Distillation Scheme with a Post-Distillation Column for
Dehydration of Aqueous Ethanol Using Ethanol Selective Solvents
FIG. 3 shows an integrated process for dehydrating aqueous ethanol
in a system that includes an EDC 241, SRC 243, and a Post-DC 242
wherein the ED solvent used preferentially extracts ethanol in EDC
241. An aqueous ethanol feed is fed via lines or streams 51 and 52
to the middle portion of extractive distillation column 241. The
feed contains typically 10 to 95 wt % ethanol, preferably 85 to 95
wt % ethanol, and more preferably 90 to 92 wt % ethanol. In a
preferred but optional embodiment, feed stream 51 is partial or
totally vaporized by the lean solvent feed to EDC 241 in cooler
244. Lean solvent from the bottom of SRC 243 is fed via lines 62
and 53 into an upper portion of EDC 241 after being cooled in
cooler 244. The lean solvent entry point in EDC 241 is selected at
a location such that there are a few trays above the solvent tray
to essentially eliminate entrained solvent that is carried over to
the EDC overhead.
In this scheme, EDC 241 is configured to produce a rich solvent
containing only solvent and ethanol in the bottom of EDC 241 and to
allow a certain amount of ethanol in the overhead aqueous stream so
that EDC 241 can be operated without liquid reflux (or minimal
flux) and under much less demanding conditions of lower S/F and
fewer separation stages. The overhead stream from EDC 241 contains
preferably no more than 20 wt % and more preferably no more than 5
wt % of the ethanol in the feed to EDC 241. Rich solvent saturated
with ethanol is withdrawn from the bottom of EDC 241 with pump 254
via line 55. A portion of the bottoms is passed through reboiler
251 and recycled to the bottom of EDC 241 via line 56 to maintain
the vapor flow in the column. The preferred operating conditions of
EDC 241 are similar to those described for EDC 201 in the process
shown in FIG. 1.
The overhead aqueous vapor stream from EDC 241 containing water and
a certain amount of ethanol is fed via line 54 to Post-DC 242.
Again, Post-DC 242 can be operated favorably to produce pure water
in the bottoms and a vapor mixture of water and ethanol in the
overhead stream, which exits via line 64 and is condensed in
condenser 245 and transferred to accumulator 247 via line 65. At
least a portion of the condensate is recycled to Post-DC 242 as
liquid reflux via line 66 by pump 249. The remaining portion of the
condensate is recycled to EDC 241 via lines 67 and 52 to recover
the ethanol. The bottoms stream of Post-DC 242 containing pure
water is withdrawn by pump 255 via line 68 and a portion is
disposed of through line 70 and the remaining portion is heated in
reboiler 252 before being recycled back into Post-DC 242 via line
69. In order to feed the overhead vapor of EDC 241 into column
Post-DC 242, EDC 241 is operated at higher pressures than that of
Post-DC 242. EDC 241 bottom (rich solvent) stream containing only
ethanol and solvent is withdrawn by pump 254 and fed into SRC 243
via lines 55 and 57. The preferred operating conditions of SRC 243
are similar to those disclosed in the description of SRC 202 in
FIG. 1, to maintain the bottom temperature at below 200.degree. C.
and preferably in the range of 160.degree. C. to 180.degree. C. to
minimize solvent decomposition.
A vapor stream containing essentially of pure ethanol exits the top
of SRC 243 via line 58 and is condensed in condenser 246. The
pressure in condenser 246 as well as at the top of SRC 243 ranges
from 50 to 500 mmHg (absolute) and preferably from 150 to 300 mmHg
(absolute), depending upon the boiling point of the solvent. SRC
243 overhead is connected to a vacuum source (not shown), which can
be a vacuum pump or steam ejector, through line 71. The condensate
from condenser 246 is transferred via line 59 to accumulator 248,
where a portion of the condensate is recycled through pump 250 to
the top of SRC 243 via line 60 as a liquid reflux to provide liquid
flow for the upper portion of SRC 243. The remaining portion of the
condensate is withdrawn from accumulator 248 with pump 250 via line
61 in the form of anhydrous ethanol.
In the meantime, lean solvent is withdrawn from the bottom of SRC
243 with pump 256 via line 62. A portion of this lean solvent is
heated in reboiler 253 and recycled to the bottom of SRC 243 via
line 63 to maintain the bottoms temperature in the desired
temperature range.
Extractive Distillation Scheme with a Pre-Distillation Column for
Dehydration of Fermentation Broth Using Water Selective
Solvents
FIG. 4 shows an integrated process for dehydration of fermentation
broth where the ED solvent used preferentially extracts water in
the EDC. The system includes a Pre-Distillation Column (Pre-DC)
261, EDC 262 and SRC 263. Fermentation broth, which typically
contains 10 to 12 wt % ethanol, is fed via lines 81 and 82 into the
middle portion of Pre-DC 261. In a preferred but optional
embodiment, feed stream 81 is initially heated by the EDC overhead
vapor that is circulated into condenser 267 and subsequently heated
by the energy from the bottoms water stream from Pre-DC 261 in heat
exchanger 264 before being partial or totally vaporized by the lean
solvent feed to the EDC through heat exchanger 266. A vaporous
side-cut stream containing 80 to 95 wt % ethanol, preferably 85 to
92 wt % ethanol, and more preferably 90 to 92 wt % ethanol is
withdrawn from Pre-DC 261 from a location in Pre-DC 261 that is
preferably 1 or 2 trays below the top and just below the liquid
reflux entry point and the vapor fed into EDC 262 as the vaporous
ethanol feed. The overhead stream in line 83 comprising vaporous
ethanol, water and essentially all the light minor components
including formaldehyde, methanol, and light esters from the top of
Pre-DC 261 passes through partial condenser 265, which is
maintained at an average temperature that is from 60 to 65.degree.
C. At this temperature range, only ethanol and water are condensed
and the condensate from accumulator 269 is recycled with pump 272
into Pre-DC 261 as liquid reflux via line 84. The vaporous light
components, which are not suitable for human consumption, exit
condenser 265 via line 86 and are subsequently condensed separately
at lower temperatures.
By using glycerin or other nontoxic ED solvents, food and medicinal
grade ethanol can be produced with this process. The bottoms stream
from Pre-DC 261 which contains essentially water is withdrawn via
line 87 by pump 278. A portion of this bottoms stream is heated in
reboiler 275 and recycled into the bottom of Pre-DC 261 via line 88
whereas the remaining portion of the bottom stream is passed
through heat exchanger 264 to heat the aqueous ethanol feed to
Pre-DC 261 and the cooled bottoms stream is sent back to a
fermentation section (not shown) via line 89. By designing the
process so that the side-cut at the top of Pre-DC 261 contains no
more than 92 wt % ethanol, the liquid reflux and number of trays in
Pre-DC 261 are substantially reduced. It is estimated that Pre-DC
261 requires approximately 2,570 KJ/L of energy as compared to
4,000 KJ/L (a 36% increase) that is required when the side cut
stream contains 95 wt % ethanol.
A vaporous side-cut from Pre-DC 261 is fed via line 85 to the
middle portion of EDC 262 whereas lean solvent from the bottom of
SRC 263 is fed by pump 280 via lines 101 and 90 to the upper
portion of EDC 262 after being cooled in heat exchanger 266. The
lean solvent entry point in EDC 262 is selected so that there are a
few trays above the solvent tray to essentially eliminate the
presence of entrained solvent, if any, from being carried over to
the overhead of EDC 262. Preferably, no liquid reflux is recycled
back to the top of EDC 262 although a reflux-to-distillate ratio
ranging from 0 to less than 0.5 can be used for non-functional
purposes, such as for knocking down any entrained solvent from the
overhead vapor stream. If necessary, the lean solvent temperature
can be maintained at 20.degree. C. and preferably 10.degree. C.
below the column temperature near the solvent tray which is
sufficient to generate internal reflux without additional energy
input. Overhead vapor exiting the EDC 262 via line 91 is condensed
in condenser 267 and then transferred into accumulator 270. An
ethanol product that is 99.5+ wt % pure is withdrawn from
accumulator 270 with pump 273 through line 92.
In order to achieve a 99+ wt % ethanol recovery with 99.5+ wt %
purity EDC 262 must be designed with a large number of separation
trays and the column would need to operate under a high
solvent-to-feed ratio. To minimize capital expenditure and reduce
energy requirements, alternatively, EDC 262 can be designed and
operated under conditions to achieve less than total ethanol
recovery. In particular, in a preferred embodiment, no more than 20
wt %, and preferably no more than 5 wt %, of the ethanol that is in
the EDC feed reaches the EDC bottoms (rich solvent) stream.
The rich solvent, which contains preferably no more than 5 wt % of
the ethanol in the EDC feed, is withdrawn from the bottom of EDC
262 and is fed via lines 93 and 95 by pump 279 as a partially
vaporized mixture into the middle portion of SRC 263. A portion of
the EDC bottoms is heated through reboiler 276 and recycled to the
bottom of EDC 262 via line 94 to maintain the vapor flow in EDC
262. The bottoms temperature of EDC 262 preferably is in the range
from 160.degree. C. to 200.degree. C. Lean solvent is withdrawn
from the bottoms of SRC 263 with pump 280 via line 101. A portion
of this lean solvent is heated in reboiler 277 and recycled to the
bottom of SRC 263 via line 102 to maintain the bottoms temperature
in the range of 160.degree. C. to 200.degree. C. and preferably in
the range of 160 to 180.degree. C. SRC 263 typically has an
operating pressure in the range of 50 to 500 mm Hg (absolute) and
preferably in the range of 150 to 300 mm Hg (absolute). Overhead
aqueous vapor stream from SRC 263 is transferred via line 96 to
condenser 268 under reduced pressure (vacuum), where the condensate
is transferred to accumulator 271 via line 97. SRC 263 overhead is
connected to a vacuum source (not shown) such as a vacuum pump or
steam injector through line 98. At least a portion of the
condensate is recycled to SRC 263 as liquid reflux by pump 274 via
line 99. The remaining portion of the condensate is recycled to
Pre-DC 261 via lines 100 and 82 to recover the ethanol.
Extractive Distillation Scheme with a Pre-Distillation Column for
Dehydration of Fermentation Broth Using Ethanol Selective
Solvents
FIG. 5 shows an integrated process for dehydration of the
fermentation broth where the ED solvent used preferentially
extracts ethanol in the EDC. The system includes Pre-DC 281, EDC
282, and SRC 283. Fermentation broth containing 10 to 12 wt %
ethanol is fed via lines 111 and 112 into the middle portion of
Pre-DC 281. In a preferred but optional embodiment, feed stream 111
is initially heated by the bottom water stream from Pre-DC 281 in
heat exchanger 284 and feed stream 111 is then heated by the EDC
overhead vapor which is sent through condenser 287 before being
partial or totally vaporized by the EDC lean solvent feed which is
sent through heat exchanger 286. Operations of the Pre-DC 281 for
concentrating the fermentation broth are the same as those
described for Pre-DC 261 in FIG. 4.
A vaporous side-cut stream containing 80 to 95 wt % ethanol,
preferably 85 to 92 wt % ethanol, and more preferably 90 to 92 wt %
ethanol is withdrawn from Pre-DC 281 from a location in Pre-DC 281
that is preferably 1 or 2 trays below the top and just below the
liquid reflux entry point and the vapor fed into EDC 282 as the
vaporous ethanol feed. The overhead stream in line 113 comprising
vaporous ethanol, water and essentially all the light minor
components including formaldehyde, methanol, and light esters from
the top of Pre-DC 281 passes through partial condenser 285, which
is maintained at an average temperature from 60 to 65.degree. C. At
this temperature range, only ethanol and water are condensed and
the condensate from accumulator 289 is recycled with pump 292 into
Pre-DC 281 as liquid reflux via line 114. The vaporous light
components, which are not suitable for human consumption, exit
condenser 285 via line 116 and are subsequently condensed
separately at lower temperatures.
The bottoms stream from Pre-DC 281 which contains essentially water
is withdrawn via line 117 by pump 298. A portion of this bottoms
stream is heated in reboiler 295 and recycled into the bottom of
Pre-DC 281 via line 118 whereas the remaining portion of the bottom
stream is passed through heat exchanger 284 to heat the aqueous
ethanol feed to Pre-DC 281 and the cooled bottoms stream is sent
back to a fermentation section (now shown) via line 119. By
designing the process so that the side-cut at the top of Pre-DC 281
contains no more than 92 wt % ethanol, the liquid reflux and number
of trays in Pre-DC 281 are substantially reduced.
A vaporous side-cut from Pre-DC 281 is fed via line 115 to the
middle portion of EDC 282 whereas lean solvent from the bottom of
SRC 283 is fed by pump 300 via lines 130 and 120 to the upper
portion of EDC 282 after being cooled in heat exchanger 286. The
lean solvent entry point in EDC 282 is selected so that there are a
few trays above the solvent tray to essentially eliminate the
presence of entrained solvent, if any, from being carried over to
the overhead of EDC 282.
Again, no liquid reflux is recycled back to the top of EDC 282
although a reflux-to-distillate ratio of 0 to less than 0.5 can be
used for non-functional purposes, such as for knocking down any
entrained solvent from the overhead vapor stream. Water vapor
containing less than 20 wt % and preferably less than 5 wt % of the
ethanol in the EDC feed exits the top of EDC 282 via line 121 and
is condensed in heat exchanger 287. The condensate is transferred
to accumulator 290 and recycled by pump 293 via lines 122 and 112
to the middle portion of Pre-DC 281 to recover the ethanol. The
rich solvent, which contains substantially pure ethanol and the
solvent, is withdrawn from the bottom of EDC 282 and is fed via
lines 123 and 125 by pump 299 as a partially vaporized mixture into
the middle portion of SRC 283. A portion of the EDC bottoms is
heated through reboiler 296 and recycled to the bottom of EDC 282
via line 124 to maintain the vapor flow in EDC 282.
Overhead vapor stream from SRC 283, which contains at least 99.5 wt
%/o ethanol, is transferred via line 126 to condenser 288 under
reduced pressure (vacuum), where the condensate is transferred to
accumulator 291 via line 127. SRC 283 overhead is connected to a
vacuum source (not shown) such as a vacuum pump or steam injector
through line 132. At least a portion of the condensate is recycled
to SRC 283 as liquid reflux by pump 294 via line 129. The remaining
portion of the condensate is removed via line 128 in the form of
anhydrous ethanol. Lean solvent is removed by pump 300 from the
bottoms of SRC 283 and a portion of the lean solvent is heated in
reboiler 297 and recycled to the bottom of SRC 283 via line 131 to
maintain the bottoms temperature in the range of 160 to 200.degree.
C. and preferably in the range of 160 to 180.degree. C.
Other non-limiting operating conditions for EDC 262 and SRC 263
described for FIG. 4, such as temperature, pressure and stream flow
are, respectively, adjusted for the operations of EDC 282 and SRC
283 as depicted in FIG. 5.
EXAMPLES
The following examples are presented to further illustrate the
preferred embodiments of this invention and are not to be
considered as limiting the scope of this invention.
Example 1
This example demonstrates that anhydrous ethanol with 99.5 wt %
purity and no solvent contamination can be produced from an aqueous
ethanol feed in a continuous EDC operating without liquid reflux at
the top of the column. Pilot plant tests were carried out in a
3-inch (7.62 cm) diameter continuous EDC which consisted of nine
(9) structurally packed sections with a chimney tray at the top of
the column. It was estimated that there were four theoretical trays
between the lean solvent and the ethanol feed entry points.
Aqueous feed containing 90.82 wt % ethanol and 9.18 wt % water was
pre-heated to 70 to 80.degree. C. and fed into the middle portion
of the EDC (4.sup.th section from the bottom). A lean solvent
containing 99.50 wt % glycerin and 0.50 wt % water was fed to the
chimney tray at the top of the column after pre-heating. The rates
of ethanol feed and glycerin solvent feed were 1.94 and 7.39 Kg/h,
respectively (the overall solvent-to-feed (S/F) weight ratio was
3.8). Allowing 0.5 wt % water in the lean glycerin feed can
significantly reduce the severity of the SRC operation. The EDC
kettle temperature was maintained at 149.degree. C. to generate a
vapor stream in the EDC. Surprisingly, the EDC column temperature
profile was within the range of 80 to 90.degree. C. even though the
boiling point of glycerin is 290.degree. C. Even without liquid
reflux at the top of the EDC, anhydrous ethanol containing 99.52 wt
% ethanol, 0.48 wt % water, and no measurable levels of glycerin
was produced from the EDC overhead, at a rate of 1.72 Kg/h. Rich
solvent containing 96.41 wt % glycerin, 1.12 wt % ethanol, and 2.37
wt % water was withdrawn from the EDC bottom at a rate of 7.79
Kg/h. Based on experimental data, ethanol recovery was determined
to be 96.2 wt %. Higher ethanol recovery can be readily achieved by
using an EDC with a larger number of separation stages.
These pilot plant results demonstrate that, even when using an EDC
with relatively few separation stages and operating with no liquid
reflux at the top the column, a glycerin/water solvent can break
the azeotrope of ethanol and water and produce anhydrous ethanol
with very high purity (99.5 wt %) and no solvent contamination.
Example 2
This example demonstrates that significant energy can be saved by
eliminating liquid reflux at the top of an EDC without sacrificing
anhydrous ethanol purity or product recovery.
Approximately 100 kg/h of aqueous ethanol at 25.degree. C. was fed
to tray 12 (counting from the top) and 350 kg/h of glycerin solvent
at 45.degree. C. was introduced to tray 3 of an EDC consisting of
13 theoretical trays (excluding the kettle). The aqueous ethanol
feed contained 90 wt % ethanol and 10 wt % water and the glycerin
solvent feed contained 99.7 wt % glycerin and 0.3 wt % water. The
EDC was operated with no liquid reflux and alternatively with a
liquid reflux ratio (R/D) of 1.0 to compare the energy requirements
of the EDC. In both cases, the column temperature increased from
78.degree. C. at the top (tray 1) to 128.degree. C. at the bottom
(tray 13), while the column pressure increased from 1.00 to 1.25
atmospheres through the top to the bottom. The EDC kettle operated
at 175.degree. C. and 1.27 atmospheres.
Under R/D of 1.0 the energy requirement of the EDC was determined
to be 2,865 KJ/L of produced anhydrous ethanol from the overhead of
the EDC. Purity and recovery of the anhydrous ethanol were 99.7 wt
% and 99.99 wt %, respectively. No detectable glycerin was found in
the ethanol product. Under similar operating conditions for the
EDC, it was found that anhydrous ethanol with 99.7 wt % purity and
99.9 wt % recovery could be produced from the overhead, without
employing any liquid reflux near the top of EDC. Again, no
detectable glycerin was found in the ethanol product. However, the
energy requirement without the liquid reflux was significantly
lower at 2,234 KJ/L (a 22% reduction).
Example 3
This ED process simulation result demonstrates that significant
energy can be saved by employing partial ethanol recovery in an EDC
and recovering the lost ethanol in a front-end pre-distillation
(beer) column. The process simulator was upgraded and validated by
the experimental data from an ED commercial demonstration plant.
Specifically, a small fraction of ethanol in ethanol feed stream to
the EDC is allowed to be lost to the bottom (rich solvent) stream
of the EDC. As shown in FIG. 4, the lost ethanol is then recycled
and recovered in a front-end pre-distillation column where the VLE
curve of ethanol/water is extremely favorable for distillation.
This novel process configuration not only reduces the overall
process energy requirements, but also allows the EDC to be operated
under relatively mild conditions.
In accordance with the process shown to FIG. 4, 12,600 kg/h of
fermentation broth containing 12 wt % ethanol and 88 wt % water is
introduced into the process via line 81 and combined with a small
recycled stream from SRC 263 (via line 100) before entering the
middle portion of Pre-DC (PDC) 261 via line 82. The side-cut (just
below the PDC reflux tray) containing approximately 90 wt % ethanol
is fed to the middle portion of EDC 262 via line 85, while the lean
solvent containing 99.7 wt % ethylene glycol (EG) and 0.3 wt %
water is fed to near the top of EDC 262 via line 90.
Solvent-to-feed weight ratio (S/F) in EDC 262 is maintained at 3.0.
The EDC is operated under conditions with no ethanol loss
(essentially 100% recovery) and with 2.5 wt % ethanol loss, to
determine the difference in overall process energy requirements for
the system which includes the PDC, EDC, and SRC process steps. The
process parameters are summarized in Table 1.
TABLE-US-00001 TABLE 1 1. Feed Flow Rate and Composition Ethanol
feed rate to PDC: 12,600 kg/h Composition of ethanol feed to PDC:
12 wt % ethanol and 88 wt % water Composition of ethanol feed to
EDC: 90 wt % ethanol and 10 wt % water Composition of solvent feed
to EDC: 99.7 wt % EG and 0.3 wt % water Solvent-to-feed weight
ratio in EDC (S/F): 3.0 2. Product Flow Rate and Composition
Ethanol product rate from EDC overhead: 1,512.45 kg/h Composition
of ethanol product from EDC overhead: 99.6 wt % ethanol and 0.4 wt
% water Water rate from the process: 11,088 kg/h Composition of
water from the process: 99.95 wt % water and 0.05 wt % ethanol 3.
Number of Theoretical Stages of the Column Pre-Distillation Column
(PDC): 50 Extractive Distillation Column (EDC): 15 Solvent Recovery
Column (SRC): 9 4. Column Operating Conditions 0% Ethanol Loss in
EDC 2.5% Ethanol Loss in EDC PDC EDC SRC PDC EDC SRC Liquid Reflux
(R/D) 1.81 1.66 1.81 1.75 0.58 1.50 Kettle Temperature (.degree.
C.) 131.6 185.2 163.5 131.6 179.6 163.4 Overhead Presure (atm) 2.2
1.2 0.26 2.2 1.2 0.26 Bottom Pressure (atm) 2.8 1.8 0.40 2.8 1.8
0.40 Reboiler Duty (KJ/sec) 1,348 905 190 1,355 479 235 Total
Process Energy (KJ/h) 8,794,800 7,448,400 Unit Product Energy
(KJ/L) 4,565 3,867
The data presented in Table 1 show that about a 15.3% reduction in
energy of the integrated process consisting of PDC, EDC, and SRC
can be achieved by lowering the ethanol recovery in the EDC to 97.5
wt % and recovering the lost 2.5 wt % ethanol in the front-end
pre-distillation (beer) column. The unit product energy for
producing anhydrous ethanol is reduced from 4,565 to 3,867 KJ/L.
The reduction of ethanol recovery in the EDC also leads to a
reduction in the liquid reflux in the Pre-DC, EDC, and SRC from
1.81 to 1.75, 1.66 to 0.58, and 1.81 to 1.50, respectively.
Example 4
The following comparison shows that the inventive process is
significantly more energy efficient than any of the current
commercial methods of anhydrous ethanol production.
Some early studies concluded that azeotropic distillation to be
more energy efficient than extraction distillation. For example,
Black et al, "Extractive and Azeotropic Distillation," Am. Chem.
Soc. Advances in Chemistry Series No. 115, 1-16 (1972), produced
anhydrous ethanol from an aqueous ethanol feed by extractive
distillation using ethylene glycol as the solvent and by azeotropic
distillation using n-pentane as the entrainer. In this comparison,
the R/D ratio of the extractive distillation column was set at 1.8,
which is equivalent to a reflux-to-feed ratio of 1.55 as described
in the article. Black et al concluded that azeotropic distillation
was more energy efficient.
This conclusion is flawed because the investigators designed the
extractive distillation column with an exceedingly high R/D ratio
of 1.8 and charged the column with an aqueous ethanol feed that
contained an unnecessarily high ethanol concentration of 85.64 mole
% or 93.84 wt %. Although azeotropic distillation requires a feed
with a composition that is near that of an azeotropic mixture of
ethanol and water, i.e., 95.6 wt %/o ethanol, extractive
distillation has virtually no restrictions on the feed composition.
This important difference means that much more energy is expended
with azeotropic distillation in the pre-distillation process. As
evidenced by the data in FIG. 1 of U.S. Pat. No. 4,559,109 to Lee
et al, in concentrating fermentation broth with 10-12 wt % ethanol
to the azeotrope level of 95.6 wt %, the first stage of raising the
concentration from 10-12 wt % to 92 wt % consumes only 47% of the
total distillation energy whereas the final stage of raising the
concentration from 92 to 95.6 wt % consumes 53% of the total
energy.
It is apparent that Black et al. did not recognized that by
concentrating the feed stock to only 90 wt %, instead of 93.8 wt %
and by operating the extractive distillation column with minimal or
no liquid reflux, extractive distillation is clearly more
economical than azeotropric distillation. An estimate of the energy
requirements for producing anhydrous ethanol from fermentation
broth confirms the significant advantages of using extraction
distillation where the feed stock has a lower ethanol concentration
over competing processes that employ a similar feed stock with a
higher ethanol concentration. The results are summarized in Table
2.
TABLE-US-00002 TABLE 2 Comparison of Energy Consumption of Leading
Ethanol Purification Technologies (Unit: KJ/Liter of Product)
Distillation from Technology 10 to 95 vol % Dehydration Total
Energy Azeotropic Distillation 3,689 2,573 6,262* PSA (Molecular
Sieves) 3,689 1,000 4,689** Inventive ED Process 2,532 1,335
3,867*** (to 90 vol %) In all cases, vapor from overhead of the
pre-distillation column is fed to the dehydration unit directly
without condensing to save the vaporization energy. *Values are
taken from the SRI report. **Values are estimated from published
data of the most advanced pressure swing adsorption (PSA) process.
***Energy requirement for the ED process with R/D of 0.58 and
partial ethanol recovery (97.5 wt %) in the EDC, which can be
further reduced when R/D is decreased to zero.
In this comparison, energy consumptions were estimated for
azeotropic distillation and adsorptive techniques, i.e., molecular
sieves. In these conventional processes, a pre-distillation step
raises the ethanol concentration from 10 to 95 vol % and,
thereafter, a dehydration step completes the process to yield the
anhydrous ethanol. For the inventive extractive distillation
process, pre-distillation requires less energy since it raises the
ethanol concentration to only 90 vol %. In this example, the
calculation is based on a design whereby the R/D ratio in the
extractive distillation column is 0.58. The energy requirement can
be substantially lowered by employing less or no reflux. Even with
R/D ratio of 0.58, the ED process of this invention requires the
lowest energy among all major commercial technologies, just 3,867
KJ/L.
The foregoing has described the principles, preferred embodiment
and modes of operation of the present invention. However, the
invention should not be construed as limited to the particular
embodiments discussed. Instead, the above-described embodiments
should be regarded as illustrative rather than restrictive, and it
should be appreciated that variations may be made in those
embodiments by workers skilled in the art without departing from
the scope of present invention as defined by the following
claims.
* * * * *